The invention concerns a method for the preparation of thin layers of tungsten or molybdenum on a highly temperature-resistant layer substrate by thermal evaporation under high vacuum.
The heavy metals, tungsten and molybdenum, are useful materials for many X-ray tubes and electron tubes because of their high melting points and high-temperature strength properties. Other applications of these heavy metals include contacts in high-voltage switchgear and for evaporators in vapor-deposition installations for high-vacuum metallizing.
In certain applications, particularly for rotary anodes of X-ray tubes, thin layers of these heavy metals on highly temperature-resistant layer substrates, such as, for instance, electro graphite or Al.sub.2 O.sub.3 ceramics, are desired. These thin layers can be prepared by costly and cumbersome methods such as fusion electrolysis, precipitation from the vapor phase of chemical compounds or by plasma spraying. Vapor deposition of thin layers on layer substrates, however, presents difficulties because of the high melting point and the lower vapor pressure of tungsten and molybdenum. Although it is possible to vapor-deposit tungsten or molybdenum by means of electron-beam evaporation or cathode sputtering, the far more economical thermal vapor deposition under high vacuum, which is easier to control as far as layer formation is concerned, fails mainly because of the high evaporator temperatures required. For example, the thermal evaporation of tungsten and molybdenum requires temperatures of between 3000.degree. and 3500.degree. C. Until recently, it has not been possible to find a suitable material for the evaporator to withstand these temperatures. (See L.I. Maissel and R. Glang, Handbook of Thin-Film Technology, McGraw-Hill, 1970, p. 1-54).